Glass ionomer cements have been in clinical use since 1975 in Europe and were introduced in the U.S. in 1977. See U.S. Pat. No. 4,209,434, issued June 24, 1980. Glass ionomer cements have been generally formulated to two different consistencies, a Type I, for luting of castings, and a Type II, as a restorative.
When introduced into America's dentistry, glass ionomer cements were publicized as the answer to some of dentistry's prayers, because they are tooth-colored, they bond to dentin and enamel, and they contain fluoride ions, which, in theory at least, minimize the occurrence of secondary caries.
The chemical basis of glass ionomer cements is a cross between the silicate cements and the polyacrylic acid cements. The powder portion of the glass ionomers is a finely powdered chemically active glass, such as an aluminosilicate glass, which has been prepared with a fluoride flux, and the liquid portion is a water solution of polyacrylic acid, polymaleic acid, polyitaconic acid, or a copolymer of at least two of these three acids. The glass dissociates into the acid, forming a gel, so that the resultant "cured" material comprises small glass particles suspended in a gel of metal salts of polycarboxylic acids. The reaction of the glass ionomer when the powder is mixed with the liquid containing polyacrylic acid is:
Aluminosilicate glass powder (with fluoride)+ (liquid) polyacrylic acid=polysalt gel matrix+ silica gel coating.
Some of the more recent developments in glass ionomers has have taken the powder portion and coated the glass with a polyacrylic acid solid. The liquid is then water containing a small percent of tartaric acid or a like compound. This formulation has made possible variations in working and setting times, dependent on the particle size of the glass powder or the addition of other reagents to the material.
The setting reaction of the glass ionomer cement can be prolonged to provide a four minute preliminary set and then a more complete setting reaction over the next hour. However, the short preliminary set limits the amount of manipulation of the material during a patient appointment.
At the preliminary stage of setting, the glass ionomer is extremely susceptible to water contamination and dehydration of its own matrix. The material either as a luting agent or a restorative must then be protected by a water resistant varnish during the set. The clinical implications of this requirement will be discussed below.
As time went on, problems inherent in the glass ionomers began to come to light. They were technique-sensitive, sometimes failing in a very short time. They became opaque with time. And, little by little, reports starting appearing about pulpal sensitivity. These reports increased to the point where, in September 1984, the American Dental Association published a precaution position paper on the glass ionomers.
An optimum glass ionomer cement has been sought by the dental industry over the past ten years because the material offers several beneficial advantages over past materials, even though several of the disadvantages have still not been completely overcome. Modifications of technique allow for the use of glass ionomers in combination with other restorative materials to overcome past deficiencies. Glass ionomers are adhesive to calcified tooth structure, both dentin and enamel. They also contain ion-leachable fluoride which affords the addition of protection against decalcification of the surrounding tooth structure to the glass ionomer due to caries.
Glass ionomer cements are not only truly adhesive to both dentin and enamel; they have also shown some adhesion to some of the non-precious alloys currently used in crown and bridge structures. When mixed and handled correctly as a luting agent, they give physical properties superior to that of zinc phosphate cement, and they are biologically compatible with the pulp. However, the biological compatibility of glass ionomer cements has sometimes been in question (JADA, Vol. 109, September 1984). Reports of sensitivity have usually been associated with the cementation of crown and bridge restorations, and the cause of such sensitivity is not known at this time. Theoretically, the post-operative sensitivity is due to the prolonged duration of set of the glass ionomer or due to the hydraulic pressures of cementation where there is minimal thickness of dentin.
Glass ionomers have a protective effect at the margins of a restoration against recurrent decay due to leachable fluoride ions. Studies have shown that this fluoride ion causes a significant decrease in the enamel solubility at the glass ionomer-tooth interface, three times more protective than silicate cements.
Improved adhesion of glass ionomers to tooth structure can be accomplished by using a 25 percent polyacrylic acid cleanser (e.g., DenMat Cavity Cleanser) for ten seconds on the dentin or enamel which is to be bonded to with the glass ionomer. The percent of polyacrylic acid and time of placement is critical to avoid problems with the unclogging of the dentinal tubules. The weak polyacrylic acid cavity cleanser is apparently acting not as an etchant but as a cleanser of loose debris in the dentin smear layer. The removal of this debris is critical for optimal bonding.
The stability of several of the calcium hydroxide restoration products on the market is currently being questioned. When replacing defective composite resin and amalgam restorations, clinicians have reported that the calcium hydroxide base is broken down to a soft consistency. Therefor, research relating to bases and liners have tended to endeavor to find more stable and adhesive dental material. One of these employs glass ionomers, and fast-setting glass ionomer liners are currently available. They have a setting time of three to four minutes and are adhesive to the cavity preparation. The cavity preparation must be moisture-free, and the glass ionomer base or liner must be applied when the surface of the material has a gloss to it. This allows for maximum adhesive bond to the dentin.
Since glass ionomer cements have biologic properties similar to the polycarboxylate cements, they can be used safely for restoring erosion lesions, and as luting agents. However, in deeper cavities it has been recommended that a base of calcium hydroxide or zinc oxide-eugenol, be used.
An additional benefit of a glass ionomer liner or base is that when it is used in conjunction with composite resin restorations it is esthetically compatible, due to the glass ionomer's translucency. The base provides for an intimate seal to the underlying dentin, and the glass ionomer itself has been etched with a gel etchant or phosphoric acid (to control the etchant on just the liner or base) for about a minute. This creates a microretentive surface that allows for an intimate seal of composite resin to glass ionomer, resulting in a restoration sealed from enamel to dentin. Its superior physical properties combined with its adhesion allow for an optimal base or liner under amalgam restorations. The fluoride affords additional protection against recurrence of decay around the base, should the restorative material become defective.
Several manufacturers have been marketing an improved glass ionomer with silver alloy powder incorporated in the glass ionomer matrix. The metal strengthens the glass ionomer without affecting the properties of adhesion to tooth structure and the protection of the leachable fluoride ions. However, silver corrodes; one only has to look at amalgam restorations which are fifty percent silver-alloy powder of the type contained in these glass ionomer mixtures and note the corrosion and weakening of the metal matrix that occurs with the corrosion process. Even when sealed within a tooth, silver shows the tendency to corrode, due to fluid flow within the dentinal tubules, In non-vital teeth, silver endodontic points exhibit this corrosion phenomenon.
Choice of the alloy powder to be added to the glass ionomer should be based on sound metallurgic and biocompatible data. A clinical evaluation of the metals used in prosthetics for removable partial dentures reveals metal alloys which are extremely corrosion resistant in the oral environment with physical properties superior to those of silver alloys. Certainly, such metals would be a better choice for enhancing the glass ionomer cements and allowing for clinical use in select instances as a core material under castings where adequate tooth structure is remaining. There is no need for extensive undercuts to retain the core material, for the glass ionomer will be adhesive.
Generally speaking, glass ionomers have not been good as a surface restorative. The glass ionomers are inferior esthetically to the composite resins. As a restorative, they are extremely sensitive to technique. Any loss of water from the material during setting causes a crazing of the restoration. Water contamination leaves the restorations opaque and chalky in appearance. Glass ionomer restorations are not polishable. The final surface texture is similar to a large particle, quartz-filled first generation composite resin.
Recently, it has been discovered that cured glass ionomers can be acid etched, allowing composite to bond to cured glass ionomers. This "ionomer sandwich" technique allows glass ionomer to be placed against the dentin, where it bonds and obturates, and allows composite to be placed over the ionomer for strength and esthetics.
When glass ionomers are etched, they can be used with the more highly esthetic composite resins to create a superior tooth-colored restoration. The ability to bond composite resin to dentin in conjunction with the dentin bonding ability of glass ionomers gives a superior restoration. The protection of the pulp with a glass ionomer and then application of the composite resin gives a superior esthetic result.
For success with glass ionomers, the following factors are important:
1. The tooth structure to be bonded should be dry and clean of all debris.
2. The glass ionomer should have a glossy appearance before application to the tooth structure.
3. Fast-setting glass ionomers, after the preliminary set, can be cut with rotary instruments in a slightly moist field. During the set of the liners, they should be coated with a water-resistant varnish.
4. Moisture should be excluded during the initial set of a glass ionomer by using a water resistant varnish.
5. A polyacrylic acid cavity cleanser, if used, should be a weak acid (25 percent polyacrylic acid, as compared to using a polyacrylic acid cement liquid which can be in excess of 50 percent polyacrylic acid) which can be applied for 10 seconds, rinsed and dried. The cut tooth surface will then have the loose debris removed without the total removal of the dentin smear layer.
The instances of postoperative sensitivity have occurred only in cases where the glass ionomer cement has been used as a luting agent, particularly for crown and bridge restorations. Also, there is some indication that, in most cases, the thickness of remaining dentin was minimal. As a result, hydraulic pressure created by cementation has been postulated as a possible cause for the sensitivity. A recent study by Pameijer and Stanley compared the pulpal response (in primates) of a glass ionomer cement with a zinc phosphate cement under continuous cementing pressure. Both cements yielded high pulpal response values with the glass ionomer cement appearing more toxic.
Additionally, the setting reaction for glass ionomer cements has been slow, taking some 30 minutes to develop a surface that is resistant to solution. The presence of moisture at this stage can also be deleterious to adequate setting. Therefore, early stress caused by adjusting occlusion or by mastication may cause fracture and subsequent microleakage. Nevertheless, the exact cause for that postoperative sensitivity has remained speculative.
It has been recommended that clinicians apply a thin coating of a lining material locally to those areas of the preparation that come closest to the pulp but not necessarily to the entire preparation. This would be particularly emphasized for preparations with minimal remaining dentin. By focusing on the areas closest to the pulp, the cement's bonding ability to enamel and dentin would not be appreciably diminished. Also, the clinician should keep in mind that proper manipulation of this cement system is critical. Therefore, proper cleaning of the tooth and casting surface, preventing moisture contamination, delaying adjustment for at least 10 minutes, and using the proper powder-to-liquid ratio are recommended to assure good bonding and to reduce the possibility of leakage and pulpal irritation.